A lack of data for smartphone games like ‘Pokémon Go’ makes it tricky to nail down revenue numbers

Mobile-game megahits may bring in more revenue than blockbuster movies, but measuring their “box-office” is a tougher task. That leaves investors playing in the dark.

Take “Pokémon Go” as an example. Its off-the-charts performance has sent Nintendo Co. shares skyrocketing this year. Yet nailing down actual revenue numbers is a tricky game. A host of third-party providers has attempted to fill in the breach, but such measurement is an...